Inkjet printing is a non-contact printing technique that is simple, inexpensive, and environmentally friendly. Due to these advantages, inkjet printing is used to print fine patterns required in various smaller and thinner apparatuses. For example, inkjet printing can be used in a process of manufacturing second-dimension planar structures, such as a process of interconnecting various electronic circuits and display substrates, or a process of manufacturing biochips, and furthermore, in a process of manufacturing three-dimensional steric structures, such as a process of manufacturing scaffolds for culturing cells in biotechnology.
In inkjet printing, the size of a droplet should be small to improve fineness, that is, the resolution of patterns printed using an inkjet printing apparatus. In general, the resolution of an inkjet printing apparatus is dependent on the size of a droplet and a spread area of a droplet in a target area, that is, an area to be printed. That is, as the size of a droplet ejected from a nozzle is smaller, the resolution is improved. Therefore, research for decreasing the size of a droplet is being actively performed.
In a conventional inkjet printing apparatus, the size of a droplet is reduced before the droplet is ejected through a nozzle. This method, however, causes many technical problems.
First, when the size of a droplet is too small, the droplet has a small cohesion volume, and thus, when ejected, the droplet can be broken into two or three droplets.
Secondly, a droplet is relatively more affected by a surface tension than by an attraction force between molecules in the droplet, which induces a cohesion force of the droplet. Accordingly, the relative attraction and adhesive forces acting between the droplet and the surface of a nozzle increase, and thus when the droplet is ejected from the nozzle, the ejection directions of the droplet become non-uniform. Therefore, the droplet is non-uniformly printed in a target area, and thus, a degree of fineness is decreased.
Thirdly, when the size of a droplet is decreased, the inertial mass of the droplet is decreased, and thus, the kinetic energy related to the inertia of the droplet is decreased. When the kinetic energy of the droplet is decreased due to the decrease in the size of the droplet, the attraction and adhesive forces acting between the nozzle and the droplet are increased and the droplet is affected more by the resistance and flow of the air when the droplet drops. Therefore, the droplet is non-uniformly printed in a target area, and thus, a degree of fineness is decreased.
Accordingly, there have been many attempts to increase the kinetic energy of a droplet so that the droplet can be precisely ejected to a target area independently from external forces, such as air resistance, while the size of the droplet is minimized. For example, a droplet can be accelerated in a nozzle before being ejected. This technique, however, necessarily requires an acceleration area in the nozzle to accelerate the droplet, which causes clogging of the nozzle since the nozzle should be formed in a fine structure to eject a fine droplet.